This is the Elmarit 90mm , one of the best portrait lenses in the general opinion.

This unit is a 3 cams model.

The conversion of the newer R-cam version is similar, only that you wouldn't have to remove any cams.

This aperture lever nose is bigger than usual, so, to remove the
black shroud, we have to focus the lens to a closer distances, then,
the black ring will be easily freed as soon we remove its three little
screws.

After that , we can unscrew the Leica-R bayonet and look for the
small ball, in the picture it remains in its place, I prefer to remove
it and keep it in secure place for the moment.

Now we have to unscrew the silver color metal cams.

This is the part you will skip if you have a newer R-Cam version .

The cams removed.

Its time to replace the small ball and leave a little bit of special
grease on it.

Here is the small ball in its place.

Install the Leica-Pentax or the Leica-Nikon bayonet, the red dot in front the scale center.

The Leica-Nikon bayonet has the red dot slightly more to the right than the one at the picture.